Replace yarn by npm

[LeBenLeBen]

To be consistent with the other repositories of the organization and allows this one to be part of the monorepo later on.

[mihkeleidast]

There's only one dependency so nothing else needs to be done, I believe. I'd rename the package to @frctl/docs and mark it as private in package.json but that can be done when importing to the monorepo.

[anitagraham]

Is there a problem with a user using yarn to install fractal?
If not, then it would be helpful to include the yarn commands as part of the install docs.
Particularly the difference between "Install Fractal" and "Install Fractal CLI"